Scientists at Southampton University found that volunteers who ate 80 grams of watercress a day - the equivalent of a single vegetable portion - had elevated levels of cancer-fighting molecules in their blood within hours of eating the salad leaves.

Writing in the British Journal of Nutrition, the authors said chemicals found in watercress, known as isothiocyanates, appeared to interfere with the growth of cancer cells.

They said: "This pilot study suggests that dietary intake of watercress may be sufficient to modulate this potential anti-cancer pathway."

The researchers added that the mechanism by which the isothiocyanates from watercress helped to inhibit cancer growth was unclear and said that further work needed to be done with larger numbers of patients to confirm their results.

The pilot study used four women, all of whom were breast cancer survivors, and monitored changes in their blood of key molecules involved in the growth of cancer cells.

The participants were asked to fast on the day of the tests and had blood samples taken before and after eating a portion of watercress.

The scientists found that six hours after they had eaten the leaves, the women experienced a drop in the activity of a molecule called 4E binding protein, which is thought to be involved in helping cancer cells survive.

Laboratory studies also showed that extracts taken from watercress leaves inhibited the growth of breast cancer cells.